Fair Values of Assets and Liabilities - Summarizes Assets Measured at Fair Value on Nonrecurring Basis (Detail) - Fair Value, Nonrecurring [Member] - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Financing Receivable Nonaccrual Fair Value Disclosure	$ 909	$ 996
Significant Unobservable Inputs Level 3
Financing Receivable Nonaccrual Fair Value Disclosure	$ 909	$ 996

Financing Receivable, Nonaccrual, Fair Value Disclosure

No definition available.

Name:	fsea_FinancingReceivableNonaccrualFairValueDisclosure
Namespace Prefix:	fsea_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant